[Rd] Modifying parsed code

[hadley wickham]

I would like to take this:

.img(plot(1:10), filename="a")

and produce

plot(1:10)

ie. whenever .img is used, I want to take the first argument and throw
away everything else.

(I am trying to produce a Sweave like environment in which I can apply
certain functions, but not have them displayed in the output)

I think I should be able to do it using substitute, but I don't know
how to operate on language objects.  Any hints would be appreciated.

Thanks,

Hadley